This invention relates to CRT or the like display systems, and more particularly to a display system having selectable automatic CRT cutoff stabilization or auto kinescope bias (AKB) with CRT feedback current simulation.
Automatic cathode ray tube (CRT) cutoff stabilization is conventionally achieved by inserting reference pulses near the video black level during the vertical blanking interval. In a closed loop manner, relative DC biases are varied to achieve equal CRT red, green and blue (RGB) cathode currents. Normally, there reference pulses are in the top overscanned part of the display and are not visible. However, in display modes in which the vertical deflection is underscanned, such as in the case where a 16:9 aspect ratio picture is displayed on a 4:3 display, these reference pulses become visible and distracting.
Referring now to FIG. 1, a schematic diagram of a conventional display system 1 with auto kinescope bias (AKB) is shown. The display system 1 includes a video processor integrated circuit (IC) 3 which monitors the current feedback on line 2 feed to the current feedback input terminal 5 during reference pulse intervals and adjusts the red (R) output bias 4R, the green (G) output bias 4G, and the blue (B) output bias 4B to maintain equal cathode currents. The display system 1 further includes a CRT output driving stage 6 having a plurality of CRT drive amplifiers 6a, 6b, 6c which are operational amplifiers. Each of the CRT drive amplifiers 6a, 6b, 6c have three outputs on lines 81, 82 and 83. The output on line 81 of a respective CRT drive amplifier 6a, 6b, 6c drives a respective red, green and blue CRT""s cathode RC, GC and BC and sink current. The output on line 82 of a respective CRT drive amplifier 6a, 6b, 6c supplies the cathode feedback current to the AKB sensing circuitry 9 of the video processor integrated circuit (IC) 3 on line 2. The output on line 83 of a respective CRT drive amplifier 6a, 6b, 6c drives a respective one of the feedback resistors R7, R8, R9.
The CRT drive amplifiers 6a, 6b, 6c each have a resistor or impedance R4, R5 and R6, respectively, coupled to a first input terminal of a respective CRT drive amplifier 6a, 6b, 6c. Moreover, feedback resistors R7, R8 and R9 are coupled to between the resistor or impedance R4, R5 and R6, respectively, and the input terminal of its respective CRT drive amplifier 6a, 6b, 6c. Furthermore, each of the CRT drive amplifiers 6a, 6b, 6c have a second input terminal which receives reference pulses near the video black level during the vertical blanking interval on line 7.
The present invention contemplates a display system which has a means for stabilizing the AKB closed loop in the absence of a CRT supplying the cathode feedback current and which applies blanking to prevent the reference pulses from being visible.
The present invention further contemplates a display system which has a means for selectively defeating AKB or, in other words, not using AKB and substituting an alternative cathode feedback current source such as during a vertical underscanned condition.